This invention relates to the use of optical fibers, and, more particularly, to the construction of a bobbin upon which an optical fiber cable is wound.
Optical fibers are strands of glass fiber processed so that light beams transmitted therethrough are subject to total internal reflection. A large fraction of the incident intensity of light directed into the fiber is received at the other end of the fiber, even though the fiber may be hundreds of meters long. Optical fibers have shown great promise in communications applications, because a high density of information may be carried along the fiber and because the quality of the signal is less subject to external interferences of various types than are electrical signals carried on metallic wires. Moreover, the glass fibers are light in weight and made from a highly plentiful substance, silicon dioxide.
Glass optical fibers are typically fabricated by preparing a preform of glasses of two different optical indices of refraction, one inside the other, and processing the preform to a fiber. The optical fiber is coated with a polymer layer termed a buffer to protect the glass from scratching or other damage, the resulting combination of an optical fiber within a buffer layer being termed an "optical cable". As an example of the dimensions, in a typical configuration the diameter of the glass optical fiber is about 125 micrometers, and the diameter of the fiber plus the polymer buffer (optical cable diameter) is about 250 micrometers (approximately 0.010 inches).
For such very fine fibers, the handling of the optical fiber and optical cable to avoid damage that might reduce its light transmission properties becomes an important consideration. The optical cable is typically wound onto a cylindrical or tapered bobbin made of aluminum or a composite material, with many turns adjacent to each other in a side-by-side fashion. To establish a base upon which the first layer of optical cable is wound, a base layer of steel wires of about the same diameter as the optical cable is first wound onto the bobbin. The optical cable is then wound onto that wire base. After one layer is complete, another layer of optical cable is wound on top of the first layer, and so on. The final assembly of the bobbin and the wound layers of optical cable is termed a canister, and the mass of wound optical cable is termed the fiber pack. When the optical cable is later to be used, the optical cable is paid out from the canister in a direction parallel to the axis of the bobbin and the canister, termed the payout axis.
It has been found by experience that, where the optical cable is to be paid out from the canister in a rapid fashion, as for example over a hundred meters per second, the turns of optical fiber must be held in place on the canister with an adhesive. The adhesive holds each turn of optical cable in place as adjacent turns and layers are initially wound onto the canister, and also as adjacent turns and layers are paid out. Without the use of an adhesive, payout of the optical cable may not be uniform and regular, leading to snarls or snags of the cable that damage it or cause it to break as it is paid out.
Although the optical cable may be wound onto the bobbin and wire base layer smoothly to form a uniform fiber pack, during prolonged storage and temperature changes the adhesively bound fiber pack can shift due to differences in the thermal expansion between the bobbin, the base layer, and the fiber pack. Instability can also result from other factors including the longitudinal forces resulting from the winding tension and the bobbin taper, which can cause the wires of the base layer to pop up. The resulting shifting of the fiber pack causes separations or slumps in the fiber pack, and the optical cable may not later pay out uniformly.
A similar problem had been encountered in prior practice wherein metallic conductor wires had been wound upon aluminum bobbins. The problem had been alleviated by forming a grooved surface upon the bobbin of the proper pitch so that the wire could be wound into the grooves. The grooves were formed by applying a curable adhesive to the surface of the bobbin, B-staging the adhesive, winding a wire around the bobbin in a spiral pattern under a very specific applied tension, and fully curing the adhesive with the wire in place. When the groove-forming wire was removed, the spiral pattern remained in the cured adhesive, and the conductor wire was wound onto the spiral. However, when this same approach was tried for the graphite-epoxy composite bobbin used with optical cables, it was not possible to form a regular spiral pattern with properly sized grooves.
There therefore exists a need for an approach for forming a fiber pack upon a composite bobbin, in a manner such that the fiber pack is resistant to separation-type failures during storage, which in turn can lead to an inability to achieve proper payout during use. The present invention fulfills this need, and further provides related advantages.